A preliminary study of hot corrosion by NaCl on Ti-48Al-2Cr-2Nb

A TiAl alloy (nominal composition Ti-48Al-2Cr-2Nb in at%) was subjected to hot corrosion by application of small amounts of simulated sea salt by aqueous means that simulate condensation in marine settings. Bare ground specimens of the alloy were sprayed with seawater solution, dried, and exposed in an air furnace at 760 degrees C for times between 0.1 and 20 ks. There was obvious attack in as Little as 0.3 ks in regions where clumped salt crystals resulted from dried salt-water spots. In such regions, oxide growth was accelerated and non-adhesive. Regions under the accelerated oxide growth were pitted heterogeneously with the alpha-2 phase preferentially attacked, even in lamellar regions. Despite the visible damage, mechanical strength as measured by 4-point bending experiments always exceeded the yield strength. A consistent but small reduction of fracture strength was observed only for specimens with the longest exposure. Na was typically observed on the oxide surface, and Cl in the metal-oxide interface. In an effort to examine the potential of salt to damage TiAl in conditions relevant to heat engines, pre-oxidized specimens (3.6 ks at 760 degrees C) were exposed in similar ways using the same application methods. Accelerated oxide growth was observed during 1.2 ks cycles at 760 degrees C for 10 cycles. Further work to examine longer time exposure conditions is needed.